Reconfigurable analog-to-digital conversion sampling of antennas for phase interferometry

ABSTRACT

All of a plurality of analog-to-digital converters (ADCs) each operating in a first mode of operation within a spectrum of interest sample a signal received at one of a plurality of antennas, with the outputs of the ADCs processed to detect signals of interest based on a threshold. For each of the plurality of antennas, a corresponding one of the plurality of ADCs operating in a second mode of operation samples signals received at the one of the antennas such that signals received are sampled at all of the plurality of antennas, with the outputs of the ADCs processed to calculate an angle of arrival for at least one detected signal of interest using phase interferometry. Bandpass or non-uniform under-sampling may be employed to sample all of the antennas at a relatively low data rate.

TECHNICAL FIELD

The present disclosure is directed in general to phase interferometryangle of arrival determination and more particularly to accurate andrapid angle of arrival determination over very wide instantaneousbandwidths.

BACKGROUND OF THE DISCLOSURE

Multiple antennas and associated receivers are often used to detect anddetermine the angle of arrival (AoA) of an E signal (e.g.,electromagnetic radiation such as radar). A difficulty arises in quicklyand accurately determining AoA over the entire spectrum of differentsignals of interest, especially radar, for which the highest frequencyof interest is typically at least nine times (9×) the lowest frequency.Similar difficulties may arise in connection with AoA determination inother situations.

SUMMARY OF THE DISCLOSURE

In an apparatus operating in a first mode, all of a plurality ofreceivers sample a signal received at at least one of a plurality ofantennas, with the outputs of the ADCs processed to detect signals ofinterest based on a threshold. The receivers may each include ananalog-to-digital converter (ADC), an ADC preceded by a filter, or anADC preceded by a frequency conversion stage and a filter. In a secondoperating mode, a corresponding ADC for each of the plurality ofantennas samples the signal within the respective band received at eachof the antennas, such that each signal within the spectrum of interestreceived is sampled at the plurality of antennas, with the outputs ofthe ADCs processed to calculate an angle of arrival for at least onedetected signal of interest using phase interferometry. Bandpass ornon-uniform under-sampling may be employed to sample each of theantennas at a relatively low data rate. In the second mode, each ADCmeasures the phase and amplitude of one or more signals at one or morefrequencies.

In one method, all of a plurality of ADCs, each operating in a firstmode of operation and within a spectrum of interest, are used to samplea signal received at one of a plurality of antennas, with the samplingbeing uniformly spaced in time. Outputs of the ADCs corresponding to thesampling of the signal at the one antenna are processed to detectsignals of interest based on a threshold. Then, for each one of theplurality of antennas, a corresponding one of the plurality of ADCs,operating in a second mode of operation within the spectrum of interest,is used to sample the signal within the respective band received at eachof the antennas such that each signal within the spectrum of interestreceived is sampled at the plurality of antennas. Outputs of the ADCscorresponding to the sampling of the signals at each of the plurality ofantennas are processed to calculate an angle of arrival for at least onedetected signal of interest using phase interferometry. In the firstmode of operation, all of the plurality of ADCs sample one of theplurality of antennas, and the sample times of each ADC are interleaved.The outputs of the ADCs corresponding to the sampling of the signalreceived at the one antenna may be interleaved before processing theoutputs to detect the signals of interest. The signals may be sampled atthe Nyquist rate, with each of N ADCs sampling at a rate of at least 1/Nof the Nyquist rate. (“Nyquist sampling” and the associated concept of a“Nyquist rate” refer to critical sampling and/or sampling withoutaliasing). At least one frequency of the signals of interest may bedetermined using the outputs of the ADCs corresponding to the samplingof the signal at the one antenna. In the second mode of operation,triggered by the detection of at least one signal in the first mode,each one of the plurality of ADCs samples a corresponding one of theplurality of antennas. The signals at each antenna may be sampled at arate well below twice the highest frequency using bandpass sampling byselecting a radio frequency filter frequency band and a clock rateproviding unaliased bandpass sampling. Phase interferometry may beperformed on at least one of frequencies for detected signals withinthat bandpass. Alternatively, in a variant of the second mode ofoperation, the signals may be sampled at an average rate of 1/N of theNyquist rate using samples spaced non-uniformly in time (non-uniformunder-sampling), although uniform under-sampling may alternatively beemployed. Phase interferometry may be performed directly on one or morestrong signals within the spectrum of interest among a plurality ofsignals producing the sampled signals. Phase interferometry may beperformed on a weaker signal among the plurality of signals by detectingand removing one or more stronger radar signals among a plurality ofradar signals from the sampled signals. At least one of new emitters andfrequency agile emitters, especially radar emitters, within the spectrumof interest may be detected after removing the one or more strongersignals. While processing the outputs of each of the ADCs to calculatethe angle of arrival for the at least one detected signal of interestusing phase interferometry, any new signals of interest within thespectrum of interest may be detected.

One apparatus includes a plurality of antennas, a plurality of ADCscommunicably coupled to the plurality of antennas, each of the ADCsconfigured to operate in at least a first mode of operation and a secondmode of operation and each configured to operate on bands within aspectrum of interest, and a processor communicably coupled to the ADCs.In the first mode of operation, all of the ADCs are configured to samplea signal received at one of the antennas, with the processor configuredto process outputs of the ADCs corresponding to the sampling of thesignal at the one antenna to detect signals of interest based on athreshold. In the second mode of operation, each of the ADCs isconfigured to sample the signal within the respective band received at acorresponding one of the antennas such that each signal within thespectrum of interest received is sampled at the plurality of antennas,with the processor configured to process outputs of the ADCscorresponding to the sampling of the signals at each of the antennas tocalculate an angle of arrival for at least one detected signal ofinterest using phase interferometry. In the first mode of operation, allof the plurality of ADCs sample one of the plurality of antennas, andthe sample times of each ADC are interleaved. The outputs of the ADCscorresponding to the sampling of the signal received at the one antennamay be interleaved before processing the outputs to detect the signalsof interest. The signals may be sampled at the Nyquist rate, with eachof N ADCs sampling at a rate of 1/N of the Nyquist rate. At least one ofa frequency, amplitude and phase of the signals of interest may bedetermined using the outputs of the ADCs corresponding to the samplingof the signal at the one antenna. In the second mode of operation,triggered by detection of at least one signal in the first mode, eachone of the plurality of ADCs samples a corresponding one of theplurality of antennas. The signals may be sampled at a rate of 1/N ofthe Nyquist rate using bandpass sampling by selecting a radio frequencyfilter frequency band and a clock rate providing unaliased bandpasssampling. Phase interferometry may be performed on at least one ofselected cued signals and all detected signals within that bandpass.Alternatively, in a variant of the second mode of operation, the signalsmay be sampled at an average rate of 1/N of the Nyquist rate usingnon-uniform under-sampling. Phase interferometry may be performeddirectly on one or more signals within the spectrum of interest among aplurality of signals producing the sampled signals. Phase interferometrymay be performed on a weaker signal at frequencies of signals detectedin the first more among the plurality of signals and new signals may bedetected and their phase computed by removing one or more strongersignals among a plurality of signals from the sampled signals. At leastone of new emitters and frequency agile emitters within the spectrum ofinterest may be detected after removing the one or more strongersignals. While processing the outputs of each of the ADCs to calculatethe angle of arrival for the at least one detected signal of interestusing phase interferometry, any new signals of interest within thespectrum of interest may be detected.

In one variant, one ADC is connected to each antenna in the first mode,each performing bandpass sampling with a separate RF filter and anindividualized sample rate (and clock) such that the ensemble covers thespectrum of interest.

Although specific advantages have been enumerated above, variousembodiments may include some, none, or all of the enumerated advantages.Additionally, other technical advantages may become readily apparent toone of ordinary skill in the art after review of the following figuresand description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 is a diagram illustrating an environment within which phaseinterferometry angle of arrival detection may be performed in accordancewith one embodiment of the present disclosure;

FIGS. 2A-2C are diagrams of a phase interferometry radar warningreceiver in accordance with embodiments of the present disclosure;

FIG. 3 is a high level flow diagram for phase interferometry radarsignal angle of arrival (AoA) determination using bandpass sampling inaccordance with embodiments of the present disclosure;

FIG. 4 is a high level flow diagram for phase interferometry radarsignal angle of arrival determination using under-sampling in accordancewith embodiments of the present disclosure;

FIGS. 5A and 5B illustrate an example of the PSD resulting fromnon-uniform under-sampling in accordance with embodiments of the presentdisclosure; and

FIGS. 5C-5E show AoA error distributions for three different modes ofoperation in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

It should be understood at the outset that, although exemplaryembodiments are illustrated in the figures and described below, theprinciples of the present disclosure may be implemented using any numberof techniques, whether currently known or not. The present disclosureshould in no way be limited to the exemplary implementations andtechniques illustrated in the drawings and described below.Additionally, unless otherwise specifically noted, articles depicted inthe drawings are not necessarily drawn to scale.

Detection of electromagnetic signals and determination of accurate angleof arrival (AoA) for those signals anywhere within a very wideinstantaneous bandwidth while simultaneously minimizing the total size,weight, power and cost (SWaP-C) at each antenna station is difficult,particularly for electronic warfare (EW) systems and especially forinterferometer arrays. New threats require electronic support (ES) andradar warning receiver (RWR) systems to capture very wide instantaneousbandwidths, especially for frequency agile emitters. Total SWaP-C isdriven, in part, by total data rate and required analog-to-digitalconverter (ADC) sample rate.

Some approaches to AoA determination could involve performinginterferometry over segments of spectrum—for example, 2-6 Giga-Hertz(GHz) or 6-18 GHz. However, limiting spectrum scanning to limited bandsin order to intercept signals of interest will have decreasing successagainst future agile threats. In these approaches, when signals ofinterest are detected, multiple (N) receivers/antennas are all cued tothe frequency of the detected signal to measure AoA. Such a systemcannot receive new or hopped signals across the spectrum while measuringAoA. Other approaches to the problem would receive the full spectrum ofinterest, but require N times the data rate, and thus N times the powerfor data conversion and input/output (I/O) compared to the presentdisclosure. Power is very limited at many antenna stations, as is theability to manage waste heat, and it is desirable to reduce or minimizethe power consumed and heat generated at such stations. Further, use ofsegmented spectral regions may involve using specific antenna spacingpatterns for each segment.

A typical phase interferometry system may use 4-6 antennas that arescanned in frequency to cover the spectrum of interest and thatcollectively perform direction-finding (DF) when one of the antennasfinds a signal of interest. The channels for each antenna receive pathmay be tuned independently over bands encompassing hundreds of MHz oreven more than a GHz, but still sense only a small portion of thespectrum of interest at any one time. Down converters within the antennareceive paths contain filter banks that parse the spectrum into bands,and precision angle of arrival determination by interferometry isperformed with the antennas tuned to a common spectral band. Whenperforming angle of arrival determination, only one portion of theentire spectrum of interest is sensed, such that agile signals canescape or new signals illuminating the aircraft are not detected.

In the approach of the present disclosure, receivers are reused indifferent combinations for very wideband detection and for directionfinding over a wideband spectrum. The resulting data collection andphase interferometer AoA system reduces SWaP-C and includes N antennasdeployed with N ADCs that can be configured in two modes: a first modein which all ADCs are connected to a corresponding one antenna to detectall signals of interest, with the ADCs each operating at 1/N of theNyquist frequency but interleaved to provide Nyquist sampling of thefull spectrum; and a second mode in which each receiver and ADC isconnected to one of the N antennas arranged as an interferometer todirection-find signals, while still detecting signals across the fullspectrum to provide coverage of “pop ups” or changes in emitterfrequency. In the second mode, the same ADCs are used with a differentsampling rate than in the first mode, possibly without any filtersbetween the respective antennas and each ADC. In the second mode, eachADC is sampled without aliasing using either bandpass sampling ornon-uniform under-sampling. Signals are detected at each frequency ofinterest (initially selected from those detected when operating in thefirst mode) to continue to track known signals. The phase for eachdetected signal of interest at each antenna is computed from thebandpass of non-uniformly spaced samples to measure AoA usinginterferometry. Alternatively, if the signals detected in the first modeare not at the same frequency in the second mode (e.g., the signal has“hopped” in frequency, the samples from N antennas can be testedindividually or in combination to detect and direction-find new or agileemitters. Overall, the approach provides wideband coverage for detectionand AoA determination, plus direction-finding via interferometry with alow data/sample rate.

FIG. 1 is a diagram illustrating an environment within which phaseinterferometry angle of arrival detection may be performed in accordancewith one embodiment of the present disclosure. Those skilled in the artwill recognize that, for simplicity and clarity, some features andcomponents are not explicitly shown, including those illustrated inconnection with later figures. In the exemplary environment 100 of FIG.1, signals 101 are transmitted by a transmitter system on a firstaircraft 102 toward a second aircraft 103. An antenna array (not shownin FIG. 1) on aircraft 103 (or, alternatively, on a land-basedinstallation) is configured to determine the AoA of the signals 101using phase interferometry operating in the manner described below.

FIGS. 2A-2C are diagrams of a phase interferometry receiver inaccordance with embodiments of the present disclosure. The exemplarysystems illustrated are used on and in an aircraft 103 to determine theAoA of radar signals, although other applications will be apparent tothose skilled in the art. Once again, those skilled in the art willrecognize that, for simplicity and clarity, some features and componentsare not explicitly shown. FIG. 2A illustrates the phase interferometryradar warning receiver 200 configured to operate in a first mode. Eachof N ADCs 201 a-201 n receives and samples a radio frequency signal froma single antenna 202 within an array of 3-5 antennas. Each ADC 201 a-201n runs as slow as 1/N of the Nyquist sampling rate (but may optionallyrun faster), but sample times of the ADCs 201 a-201 n are interleaved by1/Nth of a sample period such that the composite sampling is at theNyquist rate over a widest possible band, covering the full spectrum ofinterest. Signals may be detected across the full spectrum of interest.Filters 203 a-203 n within an optional filter bank are all in an openposition in this mode of operation (although, alternatively, they may bein a non-open position).

Each of the ADCs 201 a-201 n (and each of the filters 203 a-203 n, ifpresent and operative) is coupled to a master clock generator 204controlling the timing of sampling by the ADCs 201 a-201 n. Each of theADCs 201 a-201 n is also coupled to a high speed sensor and processordata network 205, and further connected by the network 205 to aninterleaver 206. The interleaver 206 arranges the samples received fromADCs 201 a-201 n according to a predetermined interleave pattern andforwards the resulting data to data processing module(s) 207, whichcomprise one or more processors (central processing units or “CPUs”) andassociated memory. The data processing modules(s) 207 identify signalsof interest within the outputs of ADCs 201 a-201 n.

FIG. 2B illustrates the phase interferometry radar warning receiver 200configured to operate in a second mode. The general structure of theantennas and ADCs is the same, except that only one of the N ADCs 201a-201 n operates on signals from each of N antennas 202 a-202 n (whichother systems use to scan the spectrum) in order to perform directionfinding. Each ADC is under-sampled using, for example, non-uniformunder-sampling techniques disclosed in U.S. Patent ApplicationPublication No. 2016/0049950 entitled “DEFEAT OF ALIASING BY INCREMENTALSAMPLING,” the content of which is incorporated herein by reference.Alternatively, other under-sampling techniques may be used. The optionalfilters 203 a-203 n may be open or may limit the signal passed to ADCs201 a-201 n to selected spectrum. All ADCs 201 a-201 n sample theirrespective signals simultaneously. The outputs of ADCs 201 a-201 n arerouted to bypass the interleaver 206 and provide N data streams tosignal processing modules 209 performing phase interferometry to findthe AoA of the signal of interest. Processing modules 207 and processingmodules 209 may share common circuitry. The received spectrum is alsoconcurrently tested for arrival of new signals so agile or pop-up signalemitters are not missed.

FIG. 2C illustrates the phase interferometry receiver 200 configured tooperate in the second mode in embodiments using bandpass sampling. Thisembodiment operates as described above for the embodiment of FIG. 2Bexcept that the filters 203 a-203 n (which are optional in otherembodiments) are configured to enable ADCs 201 a-201 n to bandpasssample the signal from each antenna using a clock rate of about 1/N. Ifthe full spectrum is 1-18 GHz, a single ADC would need to sample signalsabove 36 Giga-samples per second to avoid aliasing, with 30 Gsamp/secbeing typical. In an example of the present disclosure, where, forexample, N=4, then the sample rate will be 10 Gsamp/sec or less for anyfilter setting. Instantaneous frequency coverage for such an example isshown in TABLE I below.

FIG. 3 is a high level flow diagram for phase interferometry radarsignal angle of arrival determination using bandpass sampling inaccordance with embodiments of the present disclosure. The processdescribed is implemented by a phase interferometry receiver of the typeillustrated by FIGS. 2A and 2C. Although depicted as a series of steps,unless explicitly stated or inherently required (e.g., a signal cannotbe processed before being received), no implication is intendedregarding the ability to perform some portions of the processconcurrently or in a parallel or pipelined manner or regarding theparticular order of performance.

The process 300 begins with the ADCs 201 a-201 n configured to allsample a single antenna 202 (step 301), at different, interleaved timessuch that the full spectrum of interest is Nyquist sampled as discussedabove. The ADCs 201 a-201 n provide digital representations of thereceived emissions (if any) at the respective times. The digital data isreceived by a data processor 207 that calculates power spectraldensities (PSDs) for each frequency within the spectrum of interest forall received emissions (step 302). For uniformly sampled data, PSDs aretypically calculated by a Fast Fourier Transform (FFT) of the timeseries signal samples. For non-uniformly sampled data, other techniquesare disclosed in the incorporated U.S. Patent Application PublicationNo. 2016/0049950 entitled “DEFEAT OF ALIASING BY INCREMENTAL SAMPLING,”and described in other references related to non-uniform sampling. Basedon the PSDs, signals having a signal power greater than (or equal to orgreater than) a defined threshold are detected and identified, with thefrequency, amplitude and phase at the single antenna 202 determined foreach detected signal within the received radar emissions (step 303).Alternatively, the signals may be channelized (e.g., passed through apolyphaser digital channelizer) and the outputs of each channel testedversus a predetermined detection threshold.

The processing then loops over the bands (step 305) to which the fullspectrum of interest has been divided. In the example of FIG. 3,bandpass filters 203 a-203 n are set (step 306). Optionally, clocktiming within the ADCs 201 a-201 n relative to the signal from masterclock 204 is adjusted (step 307). Each ADC 201 a-201 n is sampled usingradar emissions received on all antennas 202 a-202 n (step 308). Theresulting information is used to assist in detecting any new signalswithin a band being processed by, for example, SP 209 (step 309). Phasesfor each detected signal, both previously detected signals and newlydetected signals, at each antenna 202 a-202 n are calculated (step 310),and AoA is calculated for each detection (step 311). Once AoA isdetermined or the signal is determined to have terminated, the processrepeats with step 301. Otherwise other processing may be performed, orthe process described may be restarted from the beginning.

As an example of bandpass sampling, a system covering frequencies from2-18 GHz may employ ADCs that are interleaved in the first mode. In thesecond mode, all antennas are configured to use one of the followingfilters and clocks:

TABLE I Filter Clock 2-3.9 GHz 8 G-samples/sec 3.7-7 GHz 7.2G-samples/sec 5.5-9 GHz 9.2 G-samples/sec 8-11.9 GHz 7.95 G-samples/sec11-14.5 GHz 10 G-samples/sec 14-18 GHz 9.2 G-samples/secThe sample frequency F_(S) relative to the low and high frequenciesf_(L), f_(H) and an integer k will be:

$\frac{2\; f_{H}}{k} \leq F_{S} \leq {\frac{2\; f_{L}}{k - 1}.}$

The parameters may be summarized as follows:

TABLE II f_(L) f_(H) k <F_(S) F_(S) >F_(S) 2 3.9 1 7.8 8 — 3.7 7 2 7 7.27.4 5.5 9 2 9 9.2 11 8 11.9 3 7.933333 7.95 8 11 14.5 3 9.666667 10 1114 18 4 9 9.2 9.333333

FIG. 4 is a high level flow diagram for phase interferometry radarsignal angle of arrival determination using non-uniform under-samplingin accordance with embodiments of the present disclosure. The process400 is a counterpart to the process 300 in FIG. 3, and shares theprocess steps identified by like reference characters while eliminatingthose process steps not depicted in FIG. 4. As with the process in FIG.3, the process depicted in FIG. 4 is implemented by a phaseinterferometry radar warning receiver of the type illustrated by FIGS.2A and 2B. Again, although depicted as a series of steps, unlessexplicitly stated or inherently required (e.g., a signal cannot beprocessed before being received), no implication is intended regardingthe ability to perform some portions of the process concurrently or in aparallel or pipelined manner or regarding the particular order ofperformance.

The process 400 begins with the ADCs 201 a-201 n configured to allsample a single antenna 202 (step 301) at a sub-Nyquist rate withinterleaved time samples, such that the N ADCs combined provide Nyquistsampling, covering the full spectrum of interest as discussed above. TheADCs 201 a-201 n provide digital representations of the received radaremissions (if any) in the respective frequency bands. The digital datais received by a data processor 207 that calculates PSDs within anyreceived emissions (step 302). PSDs are calculated as described above.Based on the PSDs, signals having PSD greater than (or equal to orgreater than) a defined threshold are detected and identified, with thefrequency, amplitude and phase at the single antenna 202 determined foreach detected signal within the received radar emissions (step 303).

In the example of FIG. 4, the sampling of each ADC 201 a-201 n at allantennas 202 a-202 n (step 308) is not preceded by setting bandpassfilters. In the first mode of operation, the receiver determinesfrequency and amplitude of signals with power greater than thethreshold. In a second mode of operation, one ADC is connected to eachantenna, and the ADCs are non-uniformly under-sampled. An initial testis made to confirm the presence of the frequencies detected in the firstmode of operation. For any signals detected at these frequencies, theamplitude and phase are determined at each antenna (step 401).Optionally, additional frequencies may be tested for the presence of newsignals within the full bandwidth of the spectrum of interest (step402). If such signals are detected, the phases are computed for the newsignals from each antenna (step 403). Following calculation of AoA foreach signal detection (step 311), optionally the time series of thedetected signals are reconstructed and subtracted from the measuredsamples (step 404). If total power of the remaining samples is above apre-defined threshold, then the process returns to loop through to lookat or for weaker signals (step 401). Otherwise other processing may beperformed, or the process described may be restarted from the beginning.

FIGS. 5A and 5B illustrate an example of the PSD resulting fromnon-uniform under-sampling in accordance with embodiments of the presentdisclosure. Two signals are present in the test data, both singlefrequency tones depicted by the two asterisks (circled in dashed lines)501 and 502 in FIGS. 5A and 5B. The strong signal dominating the initialPSD 503 (depicted as oval data points connected by lines) in FIG. 5Amust be removed before detecting the weak signal that can be seen inFIG. 5B. This occurs because non-uniform sampling causes the PSD to haveadditional background structure at other frequencies than the truefrequency. Thus the portion of the background 502 caused by the strongsignal 501 at 3.2 times the Nyquist frequency is stronger than the weaksignal 502 at 2.4 times Nyquist. After the time series of the strongsignal 501 is reconstructed and subtracted from the samples, a PSD 504(also depicted as oval data points connected by lines) of the resultingresidual data, shown in FIG. 5B, will allow detection of the weakersignal 502 at 2.4 Nyquist from this residual data.

Phase interferometry is a well understood process converting themeasured phases at each antenna to the correct AoA. Noise causes errorsin the retrieved AoA where the retrieved AoA is close to truth. In somecases, the noise is so high that one or more retrieved phases have anerror greater than 27c. When the phase error is larger than 2n, the AoAerror can be very large. FIGS. 5C-5E show AoA error distributions forthree different modes of operation. FIG. 5C characterizes theperformance of the bandpass sampled mode, which does not have the highbackground structure in the PSD, and therefore does not require removalof the strong signal 505 before determining the phase and AoA of theweak signal 506. In the AoA error distribution for each signal withoutefforts at removal, shown in FIG. 5C, the strong signal 505 is moreaccurately measured than the weak signal 506, with respective errors of0.01° versus 0.14°. The AoA error distribution using the non-uniformlyunder-sampled mode after strong signal 507, 509 removal beforedetermining the phase and AoA of the weak signal 508, 510 is illustratedin FIGS. 5D and 5E, which respectively depict cued removal and blindremoval. Cued removal uses the first mode Nyquist sampling to assist thestrong signal removal; blind removal relies solely on the Nnon-uniformly under-sampled ADCs to characterize and remove the strongsignal. In the case of these pure tones, as in FIGS. 5A-5B; results arevery similar. A small number of pulses have phase errors that cross 2π,giving large AoA errors. These pulses are filtered from thedistributions.

In variants of the second mode of operation, the results of operating inthe first, first mode may be used to provide a frequency cue or aprediction of the signal on which to perform direction-finding after theswitch to the second, second mode. Alternatively, a radio frequency (RF)filter may be employed. Phase interferometry may then be performed onselected cued signals or all detected signals, or on an operatorselectable one of the two. In this variant, the ability to test for newor agile emitters is limited to the bandpass selected. As anothervariant, in the non-uniformly under-sampled embodiments, the fullspectrum of interest may be received. Detection and removal of thestrongest signals is required in order to look for weaker signals, forexample by using approaches described in the above-referenced patentapplication publication. Those approaches are able to find the phase andamplitude of signals. As a result, the ability to test for new or agileemitters extends across the full spectrum, although the ability todetect weak signals may be limited by the accuracy of strong signalremoval.

In still other variants, in the first mode of operation, an ADC isconnected to each antenna, where a different RF filter is employedbetween each antenna and its respective ADC. The sampling clock for eachADC is tuned so that the ADC provides unaliased data from bandpasssampling.

As described above, the non-uniformly under-sampled embodiment for thesecond operating mode has been tested where each receiver works alone todetect and remove strong signals. The non-uniformly under-sampledembodiment for the second operating mode has also been tested where allreceivers work together to jointly detect and remove strong signals. Inany embodiment, fixed filters may be replaced with tunable filters.

The first mode determines if a signal is agile or not, and may thereforeadjust the clocks of uniform under-sampling to avoid aliasing. There arealso variants of bandpass sampling with two distinct clocks only.

Modifications, additions, or omissions may be made to the systems,apparatuses, and methods described herein without departing from thescope of the disclosure. For example, the components of the systems andapparatuses may be integrated or separated. Moreover, the operations ofthe systems and apparatuses disclosed herein may be performed by more,fewer, or other components and the methods described may include more,fewer, or other steps. Additionally, steps may be performed in anysuitable order. As used in this document, “each” refers to each memberof a set or each member of a subset of a set.

The description in the present application should not be read asimplying that any particular element, step, or function is an essentialor critical element which must be included in the claim scope: the scopeof patented subject matter is defined only by the allowed claims.Moreover, none of these claims are intended to invoke 35 USC § 112(f)with respect to any of the appended claims or claim elements unless theexact words “means for” or “step for” are explicitly used in theparticular claim, followed by a participle phrase identifying afunction. Use of terms such as (but not limited to) “mechanism,”“module,” “device,” “unit,” “component,” “element,” “member,”“apparatus,” “machine,” “system,” “processor,” or “controller” within aclaim is understood and intended to refer to structures known to thoseskilled in the relevant art, as further modified or enhanced by thefeatures of the claims themselves, and is not intended to invoke 35U.S.C. § 112(f).

What is claimed is:
 1. A method, comprising: using all of a plurality ofanalog-to-digital converters (ADCs) each operating in a first mode ofoperation within a spectrum of interest, sampling a signal received at asingle antenna of a plurality of antennas; processing outputs of theADCs corresponding to the sampling of the signal at the single antennato detect signals of interest based on a threshold; using, for each oneof the plurality of antennas, a corresponding ADC of the plurality ofADCs operating in a second mode of operation and on bands within thespectrum of interest, sampling signals received at each of the pluralityof antennas such that signals received at each of the plurality ofantennas are sampled; and processing outputs of the ADCs correspondingto the sampling of the signals at each of the plurality of antennas tocalculate an angle of arrival for at least one detected signal ofinterest using phase interferometry.
 2. The method according to claim 1,wherein sampling the signal at one antenna comprises sampling each ofthe antennas at 1/N of the Nyquist sampling rate for the spectrum ofinterest with interleaved sample times so that the resulting aggregatesampling is Nyquist, the method further comprising: interleaving theoutputs of the ADCs corresponding to the sampling of the signal at thesingle antenna before processing the outputs of the ADCs correspondingto the sampling of the signal received at the corresponding one of theplurality of antennas to detect the signals of interest.
 3. The methodaccording to claim 1, wherein processing outputs of the ADCscorresponding to the sampling of the signal at the single antenna todetect signals of interest based on a threshold further comprises:determining at least one of a frequency and amplitude of the signals ofinterest using the outputs of each of the ADCs corresponding to thesampling of the signal at the corresponding one antenna for therespective ADC.
 4. The method according to claim 1, wherein sampling thesignals received at all of the plurality of antennas further comprises:sampling the signals using bandpass sampling to facilitate detection ofnew signals in the bandpass.
 5. The method according to claim 4, whereinsampling the signals using bandpass sampling further comprises:selecting a radio frequency filter frequency band for different radiofrequency filters between each antenna and a corresponding ADC and asampling clock rate for each ADC providing unaliased bandpass sampling.6. The method according to claim 4, further comprising: performing phaseinterferometry in a second mode of operation on signals at at least oneof frequencies detected in the first mode of operation and all detectedsignals.
 7. The method according to claim 1, wherein sampling thesignals received at all of the plurality of antennas further comprises:sampling the signals using non-uniform under-sampling to facilitatedetection of new strong signals in the spectrum of interest, wherein thesignals are sampled at one of an average rate of 1/N of a Nyquist rate,a peak rate, or another rate.
 8. The method according to claim 7,wherein sampling the signals using non-uniform under-sampling furthercomprises: detecting and removing one or more stronger radar signalsamong a plurality of signals producing the sampled signals in order toperform signal detection, phase determination, and phase interferometryon weaker signals among the plurality of signals.
 9. The methodaccording to claim 1, further comprising: employing a different RFfilter between each antenna and the respective ADC; and tuning asampling clock for each ADC so that the ADC provides unaliased data frombandpass sampling.
 10. The method according to claim 1, furthercomprising: while processing the outputs of the ADCs to calculate theangle of arrival for the at least one detected signal of interest usingphase interferometry, testing additional frequencies over the spectrumof interest to attempt detection of any new signals of interest in oneor more bands corresponding to a respective one of the ADCs.
 11. Anapparatus, comprising: a plurality of antennas; a plurality ofanalog-to-digital converters (ADCs) communicably coupled to theplurality of antennas, each of the ADCs configured to operate in atleast a first mode of operation and a second mode of operation and eachconfigured to operate within a spectrum of interest; and a processorcommunicably coupled to the ADCs, wherein, in the first mode ofoperation, all of the ADCs are configured to sample a signal received ata single antenna of the antennas, wherein the processor is configured toprocess outputs of the ADCs corresponding to the sampling of the signalat the single antenna to detect signals of interest based on athreshold, wherein, in the second mode of operation, each of the ADCs isconfigured to sample signals received at one of the plurality ofantennas, and wherein the processor is configured to process outputs ofthe ADCs corresponding to the sampling of the signals at each of theantennas to calculate an angle of arrival for at least one detectedsignal of interest using phase interferometry.
 12. The apparatusaccording to claim 11, further comprising: an interleaver coupledbetween the ADCs and the processor and configured to interleave theoutputs of the ADCs corresponding to the sampling of the signal at thesingle antenna before processing the outputs of the ADCs correspondingto the sampling of the signal received at the corresponding one of theplurality of antennas to detect the signals of interest.
 13. Theapparatus according to claim 11, wherein the processor is furtherconfigured to determine at least one of a frequency and amplitude of thesignals of interest using the outputs of each of the ADCs correspondingto the sampling of the signal at the corresponding one antenna for therespective ADC.
 14. The apparatus according to claim 11, wherein theADCs are configured to sample the signals using bandpass sampling tofacilitate detection of new signals in the bandpass.
 15. The apparatusaccording to claim 14, wherein the ADCs are configured to sample thesignals using bandpass sampling by: selecting a radio frequency filterfrequency band for different radio frequency filters between eachantenna and a corresponding ADC and a sampling clock rate for each ADCproviding unaliased bandpass sampling.
 16. The apparatus according toclaim 14, wherein the phase interferometry is performed in the secondmode of operation on signals at at least one of frequencies detected inthe first mode of operation and all detected signals.
 17. The apparatusaccording to claim 11, wherein the ADCs are configured to sample thesignals using non-uniform under-sampling to facilitate detection of newstrong signals in the spectrum of interest, wherein the signals aresampled at one of an average rate of 1/N of a Nyquist rate, a peak rate,or another rate.
 18. The apparatus according to claim 17, wherein theADCs are configured to sample the signals using non-uniformunder-sampling by: detecting and removing one or more stronger radarsignals among a plurality of signals producing the sampled signals inorder to perform signal detection, phase determination, and phaseinterferometry on weaker signals among the plurality of signals.
 19. Theapparatus according to claim 11, wherein the processor is configured toemploy a different RF filter between each antenna and the respectiveADC, and tune a sampling clock for each ADC so that the ADC providesunaliased data from bandpass sampling.
 20. The apparatus according toclaim 11, wherein the processor is configured, while processing theoutputs of the ADCs to calculate the angle of arrival for the at leastone detected signal of interest using phase interferometry, testingadditional frequencies over the spectrum of interest to attemptdetection of any new signals of interest in one or more bandscorresponding to a respective one of the ADCs.